For forming sheds, power looms are as a rule provided with a plurality of heddle shafts, each of which has many heddles, arranged parallel to one another, through whose the yarn eyelets the warp yarns are passed. For forming sheds, or shedding, the heddle shafts are moved very rapidly up and down. This is accomplished by shaft drive system, which are also called shaft looms or eccentric looms. So-called eccentric looms generate the up-and-down motion of the heddle shafts from the rotary motion of a drive shaft, and high weaving speeds are attainable. However, such eccentric looms are inflexible. Only to a limited extent is it possible to create patterns or different kinds of bindings. For this reason, shaft drive systems are extensively used in which a pawl coupling is provided between a drive shaft and the eccentric element, for generating the shaft motion.
One such shaft loom is known for instance from German patent disclosure DE 697 02 029 T2. The pawl indexing mechanism located between the eccentric element and the driving shaft is switched on here for each shaft motion—that is, for an upward motion or a downward motion of the shaft, in each case for one-half of one revolution of the shaft. Such shaft looms are very flexible. However, such shaft looms cannot attain the operating speed of eccentric looms. The function of the pawl indexing mechanisms is vulnerable to wear. Increasing the operating speed, however, not only causes pawl wear but also leads to breakage of heddles and shafts.